Garry's mod is a half-life 2 mod with the following features:

• Ragdoll Poser


• Rope objects and people together


• Weld objects and people together


• Place a camera and watch it on your HUD


• Shoot 100's of props out of your RPG


• Attach manhacks and scanners to objects and corpses

Version 3 gives the ability to attach wheels to things. You can then control the speed of the wheels via the mouse wheel or the plus and minus keys on your keyboard.
Here's the changelog

• Shotgun can now be used to add motorized wheels


• Different wheel classes


• Each class can be destroyed and have their speed changed independently


• Rotation speeds can be adjusted with the mouse wheel


• Right click to change the wheel class


• Press use to adjust the direction the wheel rotates


• Reload removes wheel connections, if pressed again it removes the wheel


• Added construct map (rushed, I wanted to get it out, but better than nothing)


• You can now get the hevsuit even if you already have one (prevents showstopping bug in HL2)


• Fixed rope hang distance fault


• Fixed length constraint (rope) being too long


• Crossbow reload now deletes all created constraints


• Ragdolls now save their fixed constraint properly (weld)


• You can now pick up most enemy corpses


• Prevented player from picking up Jeep with the gravity gun (caused crash)


• Added on screen info for some entities rather than spamming them in the console


• Physcannon grab now wakes up paused objects (this might cause weird problems in the SP game)